1. Field of the Invention
The invention relates to a system for driving a multi-lamp (multiple lamps), and in particular to a system for driving a multi-lamp which can be used for a backlight (back light) in a passive display device such as a liquid crystal display device, and a method thereof.
2. Description of the Prior Art
In a general passive display device such as a liquid crystal display device used for a television or a computer monitor, a backlight unit as a non-emissive type device is needed to emit a light from its rear side. The backlight unit can be compartmentalized into a fall perpendicular emission method and an edged emission method depending on a light source. The increasing of a display size is made on the main use of the fall perpendicular emission method. A side light source in the fall perpendicular emission method is made up with a parallel arrangement of a plurality of discharge lamps. Currently, a cold cathode fluorescent lamp or an external electrode fluorescent lamp is being used as a discharge lamp.
To drive a parallel arrangement of a plurality of discharge lamps, it has been known to have many problems which can be overcame. For examples, the size increase of the discharge lamps in connection with increasing a display size results in a rise phenomenon of a driving voltage, problems with insulation durability, and so on, of the discharge lamp. It has already been known that this rise phenomenon does not make the discharge lamps to be stably driven. Therefore, an independent driving inverter module in each of the discharge lamps should be utilized in a back light unit adopted for the fall perpendicular emission method. This results in a serious affect on a price rise of the backlight unit and also an increase of unnecessary weight and size of the backlight unit.
It is, moreover, very difficult to have a uniformity of a illumination intensity over an entire luminous square in the back light unit because in each of the plurality of discharge lamps its operation is driven by a corresponding independent driving inverter module. To solve the above described problems a current balance technique capable of obtaining a uniformity of illumination intensity and of driving a parallel arrangement of the discharge lamps in a backlight unit has been suggested.
A system for driving a parallel arrangement of a plurality of discharge lamps is described in U.S. Pat. No. 6,717,372 to Wei-Hong Lin et al. and titled “Multi-Lamp Driving System”. The above disclosed system can basically drive two discharge lamps using one transformer of which a secondary winding is coupled to the parallel arrangement of two discharge lamps. Two windings having a common winding at one magnetic core is coupled to between the secondary winding of the transformer and one of two discharge lamps to control a current balance.
In a system for driving a multi-lamp, however, two or more transformers should be used to drive two or more discharge lamps. Further, characteristics of each discharge lamp and each of circuit elements are ideally not the same. Due to this fact, it is substantially difficult to make many lamps to be entirely and uniformly kept on a current balance.
It is, however, difficult to obtain a uniform current balance in this parallel arrangement. The changeable rage of most of voltages generated from a current deviation between discharge lamps exists within a partial voltage compared with the whole discharge voltage of the discharge lamp. Therefore, it is ineffective to have a current balance in the whole range of discharge voltage.
In a general passive display apparatus, the brightness of a backlight and a light intensity of the peripheral circumferences can exert an influence on the contrast of an optical image to be displayed on the passive display apparatus. Further, the contrast of an optical image to be displayed on the passive display apparatus can an act on scene characteristics of the displayed optical image, that is, the decreasing of a number of the displayed picture elements can result in a display of lower definition degree.
To solve the above problems of the passive display apparatus, new technologies have been suggested in which the brightness of the backlight can be adjusted depending on the light intensity of the peripheral circumferences and the characteristics of the displayed optical image. One of new technologies is the disclosure being directed to a high contrasted passive display device discusses in U.S. Pat. No. 5,717,422 to James L. Fergason titled “Variable Intensity High Contrast Passive Display”. The brightness intensity of a light source in Fergason's passive display device can be adjusted depending on the light intensity of the peripheral circumference and the characteristics of the displayed optical image. A brightness adjustment of a light source used in Fergason's passive display device may apply to an entire light source. Because of this, it is very difficult to realize a higher contrast in case of a partial dark scene or a partial bright scene.
In view of an electric power consumption, it looks forward to be partially low in electric power consumption during display of a dark scene by the control of a light source. During display of a partial bright scene by the control of a light source, the electric power consumption is kept to be higher due to the application of the whole rise of the brightness to a light source.
When an optical image to be displayed on the passive display device can become dark or bright under the darkness of a light source or under a mutual brightness difference of a light source by the control of a light source, the light source can be partially controlled depending on the characteristics of the displayed optical image so as to obtain a high contrast and to save an electric power consumption in effect.